GB 2 317 295 and EP 0 829 744 discloses a display which may be operated in 2D and 3D modes. FIG. 1 of the accompanying drawings illustrates the basic structure of one example of such a display in the 2D and 3D modes. In the 3D mode, the display comprises a compact extended backlight 1 disposed behind a spatial light modulator (SLM) embodied as a liquid crystal device (LCD) 2. The LCD 2 has a rear polariser 3 and a front polariser 4. In the 3D mode, the display is of the front parallax barrier type in which the parallax barrier is formed by a patterned retarder 5 formed on a substrate 6 and a polariser 7.
In the 2D mode also illustrated in FIG. 1, the polariser 7 is removed so that the parallax barrier is effectively disabled.
FIG. 2 of the accompanying drawings illustrates operation in the 3D mode. The retarder 5 comprises regions such as 8, which rotate the polarisation direction of light passing therethrough by 90°, and regions such as 9, which do not alter the polarisation of light passing therethrough. The regions 8 correspond to the slits of the parallax barrier whereas the regions 9 correspond to the opaque barrier portions between the slits.
In FIG. 2, polarisation directions in the plane of the drawing are represented by double-headed arrows whereas polarisation directions perpendicular to the plane of the drawing are represented by filled circles. Unpolarised light from the backlight 1 is incident on the input polariser 3, which substantially blocks the polarisation component perpendicular to the plane of the drawing and has a transmission axis 10 which passes the polarisation component in the plane of the drawing. The LCD 2 is of a type which is controlled so as to vary the polarisation rotation through the device with 90° rotation corresponding to maximum brightness. The transmission axis 11 of the output polariser 4 is orthogonal to the transmission axis 10 of the input polariser 3 so that the output polariser 4 transmits only light polarised perpendicular to the plane of the drawing.
Light from the output polariser 4 passing through the regions 9 has its polarisation unchanged. The polariser 7 has a transmission axis 12 which is orthogonal to the transmission axis 11 of the polariser 4 so that light passing through the regions 9 is substantially blocked and the regions 9 appear dark or opaque. Light passing through the regions 8 has its polarisation direction rotated by 90° so as to be parallel to the transmission axis 12 of the polariser 7. The polariser 7 thus transmits this light so that the combination of the patterned retarder 5 and the polariser 7 acts as a parallax barrier. In the 2D mode of the display, the polariser 7 is moved or removed so as to be out of the light path from the display to an observer. The barrier structure is thus no longer visible and light from both the regions 8 and the regions 9 is transmitted to an observer.
FIG. 3 of the accompanying drawings illustrates the appearance of the parallax barrier in the 3D mode at 15 and also illustrates the pixellated structure of the display. The LCD 2 comprises red (R), green (G) and blue (B) picture elements (pixels) arranged as rows and columns. Images for viewing by left and right eyes of an observer are arranged as interleaved columns (LR) of pixels and cooperate with the slit regions 8 of the retarder 5 to form viewing windows 16 in a plane spaced from the front of the display by the preferred viewing distance. Ideally, the light transmission profile for each window should be a “top hat” function with constant light intensity inside the window and zero light intensity outside the window. However, as shown at 18 in FIG. 3, there is some intensity variation across each window and non-zero intensity outside each window resulting in cross-talk between left and right images.
In the 2D mode of operation as shown in FIG. 4, light from the “black” regions of the retarder 5 is vertically polarised whereas that from the “slit” regions 8 is horizontally polarised. This results in different intensity distributions in the viewing plane because of the two polarisations, which sum together because they are orthogonal and do not interfere. Where the patterned retarder 5 is embodied as a patterned halfwave plate with the optic axis of each slit region 8 being oriented differently from the optic axis of each black region 9, light passing through the black regions 9 encounters only a single refractive index because of the retarder slow axis whereas light passing through the regions 8 encounters both the fast and slow axes of the retarder and thus passes through a different optical path. When a polariser is placed in the path, the polarisations are made the same and the light paths are able to interfere. However, in general, there is a relative phase shift between the paths and so adding the amplituders (rather than the intensities) does not cancel out the variation. This results in a variation in brightness in the 2D mode across the display and is undesirable.
FIG. 5 of the accompanying drawings illustrates the 2D mode of a 2D/3D display of the type disclosed in GB 2 236 728 and EP 0 887 666. The display of FIG. 5 differs from that shown in FIG. 2 in that, instead of being removed for the 2D mode, the polariser 7 is arranged with its transmission axis 12 at −45° to the transmission axis 11 of the polariser 4.
The lower part of FIG. 5 illustrates the effect of the display on the light wavefront in the 2D mode. The regions 9 have their slow axis aligned with the polarisation direction of light from the polariser 4 so that, as shown at 20, light from the regions 9 has a uniform wavefront. The polarisation direction of light passing through the regions 8 is rotated by 90° and experiences both the fast and slow axes of the regions 8. This is illustrated at 21.
The polariser 7 is oriented to pass light from the regions 8 and 9. In particular, the polariser 7 passes a portion of the light from the regions 9, which light has experienced only the slow axis of the retarder 5. However, the polariser 7 passes light from the regions 8, which light has experienced both the fast and slow axes. The polariser 7 therefore transmits a non-uniform wavefront as illustrated at 22 and, as described hereinbefore, this gives rise to non-uniform window illumination in the 2D mode.